Research On Structure Design And Preparation Of GaN Base Photocahtode

Vacuum ultraviolet detectors with the core of GaN base photocathodes are widely and potentially used in the such fields as missile approach warning,space detection,ultraviolet communication,fire monitoring and corona detection.To achieve high property GaN base photocathodes,the structure design and preparation technique are explored in this dissertation.Since the QE of the current GaN photocathode is not very satisfying,five samples are designed to optimize the cathode structure from the material and thickness of the buffer layer,doping concentration,doping profile and thickness of the emission layer.The QE is improved from 37%to about 60%by using the more thinner emission layer of 150nm and more higher doping concentration of 1018cm-3 level,along with the gradient doping structure and the varying Al fraction structure.The existing photocathode multi-information testing and evaluation system is upgraded,a UV light source and transmission testing optical path are added,the software is promoted and quadrupole mass spectrometry is applied to examine the residual gas in the ultra high vacuum chamber.Concentrated H2SO4,H2O2,de-ionized water,H2SO4,H2O2 and HC1 solution are applied to clean the GaN surface residues and Ga2O3 is confirmed as the main form of oxide residue at the GaN surface.A new method on deciding the atomically-clean surface of GaN according to the vacuum degree variation with the temperature in vacuum chamber is developed.The activation experiments of the gradient doping sample are performed with the Cs/O current ration of 1.64/1.62 and 1.83/1.62,respectively,and the effect shows that the highest photocurrent and QE are achieved when more Cs is delivered on the GaN surface at the first beginning.Although Cs alone can successfully achieve the NEA GaN photocathode,but the Cs/O co-activation can increase the QE value by about 8%on average.The activation experiments of the five samples show that the highest QE,which should be the key element to evaluate each sample,depends on the optimal Cs/O ratio,that is quite different for each sample.The intensities and locations built-in electric fields are calculated and the movement of the optical electrons under the high fields is analyzed.The increase of the drift and diffusion length is indicated as the reason to improve QE.Meanwhile,to avoid the negative electron mobility caused by extremely high electrical intensity,the surface doping concentration is calculated as 3.19×1017cm-3.The Cs/O activation processes are analyzed of graded doping and varying Al fraction GaN photocathodes,which show that the latter achieves much higher QE with the first 0 adsorption and takes shorter time to get the first QE peak.The variation of the QE profiles are studied with the optical properties of GaN and AlGaN.The stability evaluation is made of the optimized GaN structure by QEs comparison between the as-activated GaN photocathode and the cathode with 9 hours attenuation,which confirms the high stability characteristic of GaN photocathode and shows that the increase of the stability is not obvious by optimizing the uniform doping cathode structure.Reactivation with Cs can restore the original QEs effectively but the slight difference still exists from the optimal QEs.The absorptivity,reflectivity and transmissivity is tested and the activation technique is explored of the uniform doping Al0.24Ga0.76N photocathode.The QE of 7.6%at 250nm and cutoff wavelength of 315nm is achieved after activation,which shows that this sample has not entered the solar-blind area.The gradient band gap Al0.42Ga0.58N structure is designed and grown,which is characterized by the X-ray photoelectron spectroscopy.Ar+ sputtering is applied to clean the AlGaN surface and the result shows that it is very effective to remove the residual carbon,oxygen and gallium oxide at the surface.The activation result shows that the cut off wavelength of Alo.42Gao.58N photocathode is limited at about 285nm,and the higher QE is achieved to 23.7%at 235nm,which is improved by 68%than that of the uniform doping sample.The researches on the structure design and preparation of the GaN base photocathodes have very good application value and practical significance of the achievement of high property "solar-blind" ultraviolet photocathode.